Research On The Temperature And Strain Sensing Technology Based On COTDR

In this thesis the development status of the distributed optical fiber sensing technology have been summarized and the distributed temperature or strain measurement based on coherent optical time domain reflectometer(COTDR) is introduced in detail. The sensing mechanism of COTDR is analyzed theoretically and the reproductivity and restorability of the Rayleigh backscatter traces are verified by experiments. In order to resolve the existing problems of low injected power, low signal to noise ratio(SNR) and great measurement error, a method for distributed temperature or strain measurement based on multi-frequency COTDR is proposed. The feasibility and advantage of the scheme are proved by theoretical analysis and MATLAB simulation. The results show that, compared with the single-frequency COTDR, this method can effectively increase the threshold of stimulated Brillouin scattering(SBS), improve the SNR, and reduce the measurement errors. As the length of the fiber is 150 m,the distributed measurement with a spatial resolution of 1 m, a maximum temperature and strain measurement errors of 0.005 ℃ and 0.004 με can be realized.In order to study the fiber dispersion of the probe pulse transmission in COTDR, the influence of the dispersion in single mode fibers on the pulse width is analyzed theoretically and the formulas of fiber dispersion effects on COTDR system performance are deduced mathematically. The waveform changes of no initial chirp super-Gaussian pulse of third order transmitting in fibers is simulated, and the effects on the width of rectangular probe pulse is analyzed. The results show that the pulse shape of no initial chirp super-Gaussian pulse launched into the fiber no longer keeps single-peak structure, but experiences an evolution process from near rectangular, to multi-peak, and finally to single-peak. The pulse width influenced by the fiber dispersion goes through a process of narrowing firstly and then broadening. As the influence of the dispersion on the pulse width is slightly when the pulse width at ns level, the influence of dispersion on the single-frequency COTDR and the multi-frequency COTDR is so small that can be ignored.